ATTITUDE CONTROL OF NANOSATELLITES USING SHIFTING MASSES

Since the development of the CubeSat standard, the number of nanosatellites developed by universities, federal agencies and commercial companies has greatly increased as well as their mission complexity. This thesis focuses on the development of attitude control systems for nanosatellites. The central idea is to utilize the external disturbance forces as control input. Such technique is particular important for small and nanosatellites in low Earth orbit where the major disturbance torque, represented by the aerodynamic torque, limits the ability of spacecraft to maneuver and to achieve stabilization. The main contribution of this thesis is the development of a novel attitude control technique that uses active variation of the aerodynamic torque through center of mass shifting. By moving three shifting masses, the distances between the spacecraft's center of mass and the center of pressure of the spacecraft external surfaces can be modified, which results in changes in both magnitude and direction of the total aerodynamic torque acting on the satellite. In this fashion, the shifting masses can convert undesired aerodynamic disturbance into a useful control torque to stabilize spacecraft. In contrast to other actuators, the proposed attitude control system based on shifting masses is more efficient in environment with high disturbance, e.g., low Earth orbit. To achieve three-axis stabilization, a nonlinear adaptive feedback control is developed. The stability of the closed-loop system is analyzed using Lyapunov stability theory and demonstrated through simulations. The secondary contribution of this thesis is the application of the center of mass relocation to develop a novel automatic mass balancing system for spacecraft simulators. Spacecraft three-Axis simulators provide frictionless and, ideally, torque-free hardware simulation platforms that are crucial for validating spacecraft attitude, determination, and control strategies. To reduce the gravitational torque, the distance between the simulator center of mass and the center of rotation needs to be minimized. This work proposes an automatic mass balancing system for spacecraft simulators, which utilizes only the three shifting masses during the balancing process, without need of further actuators. The proposed method is based on an adaptive nonlinear feedback control that aims to move, in real-time, the center of mass towards the spacecraft simulator's center of rotation. The stability of the feedback system and the convergence of the estimated unknown parameter (the distance between the center of mass and the center of rotation) are analyzed. The proposed method is experimentally validated using the CubeSat Three-Axis Simulator at the Spacecraft Robotics Laboratories of the Naval Postgraduate School